1
Pavel Uher#, 2Peter Koděra,
3
Jaroslav Lexa, 1Peter Bačík
1 Dept. of Mineralogy and Petrology, Comenius University, Mlynská dolina G, 842 15 Bratislava,
Slovakia, # [email protected]
2 Dept. of Mineral Deposits, Mlynská dolina G, 842 15 Bratislava, Slovakia
3 Geological Institute, Slovak Academy of Sciences, Dúbravská cesta 9, 840 05 Bratislava, Slovakia
Key words: biotite, annite, phlogopite, halogen-rich mica, Au-porphyry deposit, saline brine, altered intermediate rocks
INTRODUCTION
Halogen anions (F and Cl) substitute hydroxyl (OH) or oxygen in several groups of rock-forming silicate minerals, e.g. in tourmalines, amphiboles and micas. Fluorine-rich to F-dominant members of the mica group occur in geological environments with high F activity, especially in highly-fractionated granites and granitic pegmatites. Chlorine-rich micas (> 1 wt. % Cl) are generally very rare in natural conditions, they were described only in some specific assemblages, e.g. in some skarns, gneisses, metamorphic banded iron formation rocks, and metasomatized volcanic or volcano- clastic rocks with hydrothermal overprint (e.g., Tracy 1991; Oen and Lustenhouwer 1992; Nash and Connor 1993; Léger et al. 1996). In our contribution, we describe a composition and possible origin of unique halogen-rich members of biotite solid solutions in altered Miocene andesite to diorite porphyry from the Detva, Biely Vrch Au-porphyry deposit in the Javorie stratovolcano, central Slovakia.
RESULTS
Biotite form irregular aggregates (20 to 300 μm across) of anhedral crystals, rarely pseudomorphs after amphibole or pyroxene in quartz veinlets or in altered groundmass of the host rock. Biotite associates with fluorapatite, rutile, ilmenite, magnetite and hercynite, it is partly replaced along rims by chamosite and clay minerals (illite, hydrobiotite or vermiculite). Two polished thin sections from the boreholes DVE-1 (depth 58.5 m) and DVE-4 (depth 66.4 m) were investigated in detail by optical study, BSE and electron microprobe. Biotite formulae were calculated on the basis of 12 anions. Four basic compositional and genetic types of biotite could be recognized:
(1) Annite to phlogopite (DVE-1 OH with increased contents of Cl (1.8-5.5 wt. %; 0.25-0.76 apfu) and F (1.0-3.1 wt. %; 0.26-0.76 apfu);
(2) Cl-dominant analogue of annite with the highest concentrations of Cl (5.9- 7.45 wt. %; 0.85-1.08 apfu) and mildly enriched contents of F (0.9-1.6 wt. %; 0.24-0.43 apfu);
147 (3) Phlogopite to fluorophlogopite with lower concentrations of Cl (0.6-1.1 wt. %; 0.07-0.15 apfu) but with the highest F enrichment (3.6-4.9 wt. %; 0.82-1.13
apfu);
(4) Annite to siderophyllite with lower concentrations of Cl (1.2-2.4 wt. %; 0.15- 0.32 apfu) and moderately increased F (2.6-3.1 wt. %; 0.63-0.78 apfu). Moreover, Cl-rich biotites (type 1 and 2) show high concentrations of Ba (up to 4.4 wt. % BaO;
0.15 apfu) and Ti (max. 3.1 wt. % TiO2;
0.20 apfu). Due to larger ionic radius of Fe2+ and Cl vs. Mg and F, strong positive correlations of Fe vs. Cl and Mg vs. F are observed (Fe-F and Mg-Cl avoidance). Compositional variations indicate following substitutions: MgFe2+-1, ClF-1,
TiO2(Fe2+,Mg)-1(OH,F)-1, and BaTAlK-1Si- 1.
CONCLUDING REMARKS
Biotites from the Detva, Biely vrch Au- porphyry deposit reveal unique compositions with presence of the new member of the mica group: Cl-dominant analogue of annite. The “chloro-annite” with up to 7.45 wt. % Cl attains the highest content of this element ever reported in the mica group (Cl-dominant analogue of ferrokinoshitalite from Sterling Hill skarn, N. Jersey contains max. 7.15 wt. % Cl, Tracy 1991; annite from Nora, Sweden shows 5.5 wt. % Cl). Beside of biotites, amphibole-group minerals from deeper part of the Detva, Biely Vrch deposit locally also show distinctly K, Fe, Cl-rich compositions. They represent a complex solid solution between hastingsite, potassic-hastinsite, a new Cl-analogue of hastingsite, and the most widespread chloro-potassichastingsite with up to 5.0 wt. % Cl ( 1.43 apfu). Fluorapatite to chlorapatite, rarely, F-rich titanite ( 2.8 wt. % F), and fluorite have been also detected. Moreover, exceptional X-site vacant, Mg, Al-rich oxy-tourmaline member attains here up to 0.45 wt. % Cl (the highest Cl content ever reported in the
tourmaline supergroup), and a unique grain of Ca2(PO4)Cl phase (“chlor-spodiosite”)
with 16.5 wt. % Cl have been identified. The recently discovered Detva, Biely Vrch porphyry gold deposit (34 t Au at 0.8 g/t Au) shows an exceptionally low Cu/Au ratio (0.018 wt. % Cu/ppm Au). It evolved in very shallow level of 500 to 1000 m bellow the paleosurface. Due to a low pressure environment, this Au-deposit was formed from nearly anhydrous Fe-K-Na-Cl salt melts containing ~ 10 ppm Au, coexisting with hydrous vapor of very low density (Koděra et al. 2014). Interaction of these fluids or melts with host andesite to diorite porphyry in the temperature range 725-400 ºC created high-temperature Na- Ca and K-silicate alterations, including precipitation of minerals with extremely high Cl and/or F contents.
Acknowledgements: Support by the APVV 0537-10 project is acknowledged.
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